1. Field of the Invention
The present invention relates to a method and system for lossless transmission of mobile IP packets, and more particularly, to a method and system for transmitting mobile IP packets without loss, in which IP packets stored in buffers of an old access control router and an old radio access station are transmitted to a new radio access station via a tunnel upon handover of a mobile terminal, so that the packets are transmitted to the mobile terminal without loss.
2. Description of the Related Art
World Interoperability for Microwave Access (WiMAX) is a technique that is being developed as a portable Internet standard by Intel Corporation. The WiMAX system supplements an existing wireless LAN 802.11a/b/g technique to greatly extend an Internet use area to outside a building.
Meanwhile, a Wireless Broadband Internet (WiBro) system is a service capable of providing long-range communication, such as portable telephones, as well as Internet data communication, such as a wireless LAN. The WiBro system may be called a portable Internet. The WiBro system is being developed as a portable Internet standard in South Korea. The WiMAX system and the WiBro system are both used as the same basic technique because they provide the portable Internet service, and are both based on Orthogonal Frequency Division Multiplexing (OFDM).
To provide the portable Internet, the WiMAX network needs to support a mobile IP. The mobile IP is a technology for a mobile terminal that uses an Internet Protocol (IP), which is a network layer protocol responsible for delivery of a connectionless datagram to a destination. A basic concept of the mobile IP is that a mobile terminal registers an address acquired on a current communication network in a home agent (HA) when accessing a remote communication network, and a home communication network sends data to the terminal located in the remote communication network. A mobile IP standard includes two parts: a mobile IP agent and a mobile node.
The mobile IP agent provides a virtual routing service to route packets to a registered mobile terminal.
The mobile node is a mobile host or a router for changing a connection point to another partial network without changing the IP address in a mobile IP network. The mobile node includes a router for transmitting an IP packet to a destination, and a host for generating and receiving packets. Examples of the host include a desktop personal computer (PC), a notebook PC, a personal digital assistant (PDA), and the like.
Currently, a concept of an IP address is used as an identifier for identifying nodes connected to the Internet. The IP address indicates a location of a node having that address. The Internet efficiently performs IP address management and packet delivery by hierarchically allocating addresses using a concept of a subnet. The subnet divides the IP address into two portions: an upper identifier used as a network identifier and a lower identifier used as an identifier for identifying nodes belonging to the network.
As a protocol defined to transmit an IP data to a mobile node moving between subnets, a mobile IP protocol manages the IP address, as follows:
First, two IP addresses are allocated to a mobile node. One address is a home address (HoA) serving as an identifier for identifying the node, and the other address is a care-of-address (CoA) that is an IP address indicating a current connection location. The mobile IP protocol allocates a new CoA each time the mobile node moves to another subnet to update current location information of the mobile node.
In this case, a service access point is provided to an upper transport layer using the HoA and a port number, so that a change resulting from movement to another subnet is invisible to an application program.
A service access point (SAP) refers to a point at which an upper layer accesses a lower layer to receive a service from the lower layer in a control operation between the adjacent layers in open system interconnection (OSI). The SAP exists on a boundary between the layers. For example, an N service access point refers to a point at which an N+1 entity in an N+1 layer accesses an N layer to receive an N service.
Since the service access point that the network layer provides to the upper layer, i.e., the transport layer, consists of the HoA and the port number, an application layer cannot recognize a subnet address after the mobile node moves to another subnet. Accordingly, the application layer cannot recognize a change resulting from the movement.
FIG. 1 illustrates a configuration of a mobile WiMAX network. The mobile WiMAX network is an OFDM-based wireless data transmission system conforming to the IEEE 802.16e standard. The mobile WiMAX network is a communication network capable of providing a seamless network connection service to a user who is moving.
The mobile WiMAX network includes a connectivity service network (CSN) 110 for providing IP-based connectivity service in an infrastructure network, and access service networks (ASNs) 120 and 130 for providing wireless access service. The access service networks 120 and 130 include radio access stations (RASs) 124, 126, 134 and 136 and access control routers (ACRs) 122 and 132. The RASs 124, 126, 134 and 136 manage a wireless resource, generate an IEEE 802.16e-based MAC frame, and schedules wireless packets. The ACRs 122 and 132 provide an IP network connectivity function and an IP router function.
A home agent (HA) 140 is a router in a subnet to which a home address of a mobile terminal 150 belongs. A foreign agent (FA) is a virtual router on a foreign network that the mobile terminal 150 in a mobile IP network visits. The mobile terminal 150, when has moved to the foreign network, obtain a care-of-address (CoA) via the FA and registers the same in the HA, the HA encapsulates a datagram directed to the mobile terminal 150 and transmits the resultant datagram to the CoA, and the FA decapsulates the datagram and transmits the same to the mobile terminal 150. The FA also serves as a default router for the mobile terminal 150 for the datagram transmitted by the mobile terminal 150.
The mobile terminal 150 is hand-held by a user, in the Mobile WiMAX network. A correspondent node (CN) 160 is a node on the network that is in communication with the mobile terminal 150. In this case, the CN 160 may not support a mobile IP protocol, but must be able to communicate with the mobile terminal 150.
FIG. 2A illustrates a mobility binding list managed by the HA, and FIG. 2B illustrates a visitor list managed by the FA.
The FA, which is a router directly connected to the node, and the HA 140, which is a router in a subnet to which the HoA belongs, build a mapping table to manage information including the CoA and the HoA.
A table managed by the home agent 140 is called as a mobility binding list 210. As shown in FIG. 2A, the mobility binding list 210 includes a mobile terminal's HoA field 212, a CoA1 field 214, and a lifetime (LT1) field 216. The HoA field 212 stores HoA information of the mobile terminal, and the CoA1 field 214 stores CoA information, which is an IP address indicating a current connection location of the mobile terminal. The LT1 field 216 stores an expiration time for keeping the above information up.
Meanwhile, a table managed by the foreign agent is referred to as a visitor list 220. Referring to FIG. 2B, the visitor list 220 includes a mobile terminal's HoA field 222, a HA field 228, a CoA2 field 224, and a LT2 field 226. The visitor list 220 further includes an HA field 228, unlike the mobility binding list 210 shown in FIG. 2A.
FIG. 3 illustrates a conventional handover procedure between different access control routers in a WiMAX network.
The mobile WiMAX network must provide a network connection service even when a subscriber carrying a mobile terminal moves to other locations. The mobile terminal measures intensity of a signal transmitted from nearby radio access stations. When the mobile terminal discovers a radio access station transmitting a signal with a greater intensity than an old radio access station, the mobile terminal changes a connection point with the network, which is called a handover.
A handover procedure between different radio access stations in the same access control router is performed according to IEEE 802.16e and WiMAX Forum NWG Stage 2/3 standard procedures. For example, in FIG. 1, the mobile terminal 150 performs the above procedure to perform the handover from the RAS 1a 124 to the RAS 1b 126, or from the RAS 2a 134 to the RAS 2b 136.
A handover procedure between different access control routers is performed according to a procedure as shown in FIG. 3. In FIG. 3, it is assumed that the mobile terminal 150 performs handover from the RAS 1b 126 belonging to the ACR 1 122 to the RAS 2a 134 belonging to the ACR 2 132.
The mobile terminal 150 moves between radio access stations belonging to different access control routers, and induces the handover procedure (S301). In this case, the RAS 1b 126 and the RAS 2a 134 complete a link-layer handover procedure between the radio access stations according to the IEEE 802.16e and WiMAX Forum NWG Stage 2/3 procedures (S302).
The mobile terminal 150 performs an agent discover procedure defined in the mobile IP protocol to obtain a CoA address (S303). The mobile terminal 150 performs an agent discover in order to examine whether the mobile terminal 150 is currently in the home network or the foreign network. If it is determined that the mobile terminal 150 is in the foreign network, the mobile terminal 150 obtains the CoA address from the FA.
The mobile terminal 150 transmits a mobile IP registration request message including the HoA address information and the CoA address information to the access control router 2 132 (S304). An access control router performs an FA function. Accordingly, the access control router 2 132 updates its visitor list by referring to the HoA address information and the CoA address information included in the registration request message.
The access control router 2 132 transmits the received mobile IP registration request message to the HA 140 (S305). Upon receipt of the registration request message, the HA 140 updates its mobility binding list using the CoA information in the received registration request message.
In response to the registration request message, the HA 140 generates a mobile IP registration response message and transmits the same to the access control router 2 132 (S306). Upon receipt of the registration response message, the access control router 2 132 transmits the message to the mobile terminal 150 to complete a new CoA registration procedure (S307).
FIGS. 4A, 4B and 4C illustrate paths for a packet transmitted from the CN to the MS before, during, and after conventional handover between access control routers. In FIGS. 4A to 4C, it is assumed that the handover is performed from an old RAS belonging to an old ACR to a new RAS belonging to a new ACR.
FIG. 4a illustrates the packet delivery path before the handover. Before the handover between the access control routers occurs, the CN 160 generates an IP packet having the HoA of the MS 150 as a destination address and transmits the same to the network (step A). When this packet is transmitted to the HA 140, the HA 140 creates a new IP header having an old CoA address of the MS 150 as a destination address using an IP tunneling technique, so that the packet is routed to the old ACR (step B). The old ACR removes the IP tunnel header generated by the HA 140 and transmits the packet to the old RAS via a tunnel (e.g., a GRE Tunnel) (step C). Upon receipt of the packet, the old RAS removes the tunnel header again and transmits an IP packet having the HoA as a destination address to the MS 150 (step D), so that the MS 150 normally receives the packet.
FIG. 4B illustrates the packet delivery path during the handover. During the handover between the ACRs, a packet is subject to IP-in-IP tunneling through an old CoA until a registration request message from the MS 150 arrives at the HA 140. Accordingly, the packet is transmitted to the old ACR (step B). Since the old ACR retains information on the MS 150 in the visitor list until the lifetime expires, the old ACR removes an IP tunnel header generated by the HA 140 and transmits the packet to the old RAS via a tunnel (step C). Upon receipt of the packet, the old RAS transmits an IP packet having a HoA in the tunnel as a destination address to the MS 150 (step D). However, since the MS 150 has been disconnected from the existing RAS, i.e., the old RAS, all packets directed to the MS 150 are discarded during this period.
FIG. 4C illustrates the packet delivery path after the handover. When handover between the ACRs is terminated and a packet is transmitted to the HA 140, the HA 140 creates a new IP header having a new CoA address as a destination address, so that the packet is routed to the new ACR (Step B′). The new ACR removes the tunnel IP header created by the HA 140 and transmits the resultant packet to the new RAS via a tunnel (step C′). Upon receipt of the packet, the new RAS transmits an IP packet having a HoA in the tunnel as a destination address to the MS 150 (step D), so that the MS 150 normally receives the packet.
The packets directed to the terminal are transmitted to the old access control router from the point that the terminal is disconnected from the old access control router to the point that the mobility binding list of the HA is updated in a conventional mobile IP protocol. However, since the terminal is disconnected from the old access control router, the packets are discarded.
It takes a time to register new address information of the terminal in the HA. If such a delay time is longer, a greater amount of packets is discarded. The loss of the packets greatly reduces a size of a congestion window in the transport layer, which degrades yield upon the handover.